1. Field
Methods and apparatuses consistent with exemplary embodiments relate to aligning a polarization-maintaining optical fiber, and more particularly, to aligning a polarization-maintaining optical fiber using a Fourier series.
2. Description of the Related Art
Polarization-maintaining (PM) optical fibers are widely used for many types of photonic assemblies. A PM optical fiber is a type of optical fiber in which the polarization of linearly-polarized light is maintained during propagation of the light through the optical fiber. One type of PM optical fiber induces stress in the core by using a cladding with a non-circular cross-section or rods of another material within the cladding. For example, FIG. 1(a) shows a cross-section of a Panda fiber 10, FIG. 1(b) shows a cross-section of an elliptical-clad fiber 12, and FIG. 1(c) shows a cross-section of a bow-tie fiber 14. Each of the optical fibers shown in FIGS. 1(a)-1(c) includes a stress-applying part (SAP) within the cladding. Another type of PM optical fiber uses the non-circular geometry of the core to maintain the polarization of the light transmitted therethrough. For example, FIG. 2(a) shows a cross-section of an elliptic-core fiber 16 produced by modified chemical vapor deposition (MCVD), and FIG. 2(b) shows a cross-section of an elliptic-core fiber 18 produced by outside vapor deposition (OVD).
PM optical fibers, like other types of fibers, are aligned and spliced together to enable communication over long distances. Fully automatic alignment and splicing has been possible for most PM fibers by using existing methods. However, each of the currently existing methods has limitations and drawbacks. Furthermore, there has been a proliferation of new specialized PM fibers in recent years, which are used for sophisticated sensor and fiber laser applications. These new specialized PM fibers are difficult to align using any of the currently existing methods.
One currently existing method of aligning PM optical fibers is an interrelation profile alignment (IPA) method. In this method, a transverse view of the fiber is used with a focus position intersecting the fiber, similar to the profile alignment system (PAS). At this focus position, there is a great deal of information that can be analyzed in the center region of the brightness intensity profile, as in the case of PAS (and unlike in the case of polarization observation by lens effect (POL), in which only contrast information is obtained). However, unlike PAS, in which the fiber is simply rotated until a certain image is obtained (such as an image in which two brightness features have symmetrical intensity and position), in the IPA method, features are plotted relative to fiber rotational position in a manner similar to the POL plotting of contrast versus rotation angle. More detail on the IPA method is contained in U.S. Patent Application Publication No. 2010/0209049 and the document entitled “Interrelation Profile Analysis Method for Alignment of Polarization-Maintaining Fiber” by Wenxin Zheng, Doug Duke, Toshiki Kubo, and Bryan Malinsky, the contents of which are incorporated herein by reference.
In summary, the current IPA technique includes performing the following operations.
First, IPA profiles are obtained from left and right fibers (fibers to be aligned and spliced together) by rotating the fibers 360 degrees and using image processing software to compute IPA values during the rotation.
Then, angle offsets of the left and right fibers are calculated using linear correlation methods. Two linear correlation methods are commonly employed: the direct correlation method and the indirect correlation method. In the direct correlation method, the left IPA profile is correlated directly with the right IPA profile by rotating IPA data to search for the maximum correlation angle. In the indirect correlation method, the left and right IPA profiles are respectively correlated to a stored IPA profile database.
Then, the left and right fibers are rotated to a desired angle offset based on the calculated angle offsets obtained from the direct or indirect correlation methods, and are spliced together.
Finally, after the left and right fibers are spliced together, the fibers are rotated again and analyzed to confirm that the operation has been successful.
However, the current IPA method suffers from the following weaknesses. First, the direct correlation method only works for similar left and right fibers. Secondly, the indirect correlation method requires preloaded IPA profiles. Third, for unknown PM optical fibers, a polarization extinction ratio (PER) meter is necessary to learn new IPA profiles. Fourth, for asymmetrical PM structures, the current IPA method does not provide a way to make automatic corrections.